1. Field of the Invention
This invention relates to a radiation image recording and read-out apparatus for exposing stimulable phosphor sheets to a radiation passing through object to have radiation image of the object stored therein, exposing the stimulable phosphor sheet to stimulating rays which cause them to emit light in proportion to the stored radiation energy, and detecting and converting the emitted light into electric signals. This invention particularly relates to a radiation image recording and read-out apparatus in which the stimulable phosphor sheets are circulated and reused for recording radiation images.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet comprising the stimulable phosphor is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays which cause it to emit light in proportion to the radiation energy stored. The light emitted from the stimulable phosphor sheet when the sheet is exposed to the stimulating rays is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image having an improved quality, particularly a high diagnostic efficiency and accuracy. The finally obtained visible image may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube (CRT). In this radiation image recording and reproducing system, the stimulable phosphor sheet is used to temporarily stored the radiation image in order to reproduce the final visible image therefrom in a final recording medium. For economical reasons, therefore, it is desirable that the stimulable phosphor sheet be used repeatedly.
Further, with regard to a mobile X-ray diagnostic station such as a traveling X-ray diagnostic station in the form of a vehicle like a bus which is provided with a radiation image recording and read-out apparatus for use in the aforesaid radiation image recording and reproducing system and moves from place to place to record radiation images for mass medical examinations, it is disadvantageous to load such a mobile X-ray diagnostic station with a number of stimulable phosphor sheets, and the amount of the stimulable phosphor sheets which can be loaded on the mobile X-ray diagnostic station is limited. Therefore, it is desired to load the mobile X-ray diagnostic station with stimulable phosphor sheets which can be used repeatedly, store the radiation images of the objects in the stimulable phosphor sheets, transfer the electric image signals read out from the stimulable phosphor sheets into a recording medium having a large storage capacity, such as a magnetic tape, and circulate and reuse the stimulable phosphor sheets for further image recording and read-out operations, thereby to obtain the radiation image signals of many objects. Further, when image recording is conducted continuously by circulating and reusing the stimulable phosphor sheets, it becomes possible to increase the image recording speed in mass medical examinations. This is very advantageous in practical use.
In order to reuse stimulable phosphor sheets as described above, the radiation energy remaining in the stimulable phosphor sheet after it is scanned with stimulating rays to read out the radiation image stored therein should be eliminated or erased by the method as described, for example, in Japanese Unexamined Patent Publication No. 56(1981)-12599 or U.S. Pat. No. 4,400,619. The stimulable phosphor sheet should then be used again for radiation image recording.
From the aforesaid viewpoint, the applicant proposed in Japanese Patent Application No. 58(1983)-66730 a built-in type radiation image recording and read-out apparatus comprising:
(i) a circulating and conveying means for conveying at least one stimulable phosphor sheet for recording a radiation image thereon along a predetermined circulation path, PA1 (ii) an image recording section positioned on said circulation path for recording a radiation transmission image of an object on said stimulable phosphor sheet by exposing said stimulable phosphor sheet to a radiation passing through said object, PA1 (iii) an image read-out section positioned on said circulation path and provided with a stimulating ray source for emitting stimulating rays for scanning said stimulable phosphor sheet carrying said radiation image stored therein in said image recording section, and a photoelectric read-out means for detecting light emitted from said stimulable phosphor sheet scanned with said stimulating rays to obtain an electric image signal, and PA1 (iv) an erasing section for, prior to the next image recording on said stimulable phosphor sheet for which the image read-out has been conducted in said image read-out section, exposing said stimulable phosphor sheet to erasing light to release the radiation energy remaining in said stimulable phosphor sheet, whereby said stimulable phosphor sheet is circulated through said image recording section, said image read-out section and said erasing section and reused for radiation image recording. PA1 (i) a circulating and conveying means for conveying stimulable phosphor sheets for recording a radiation image therein along a predetermined circulation path, PA1 (ii) an image recording section positioned on said circulation path for recording a radiation image of an object in each of said stimulable phosphor sheets by exposing said stimulable phosphor sheet to a radiation passing through said object, PA1 (iii) an image read-out section positioned on said circulation path and provided with a stimulating ray source for emitting stimulating rays for scanning said stimulable phosphor sheet carrying said radiation image stored therein at said image recording section, and a photoelectric read-out means for detecting light emitted by said stimulable phosphor sheet scanned by said stimulating rays to obtain an electric image signal, PA1 (iv) an erasing section positioned on said circulation path for, prior to the next image recording in said stimulable phosphor sheet for which the image read-out has been conducted at said image read-out section, exposing said stimulable phosphor sheet to erasing light to release the radiation energy remaining in said stimulable phosphor sheet, PA1 (v) an input means for inputing the kind of recording and the exposure conditions of said radiation, and PA1 (vi) a control means for determining the read-out condition based on the input information given by said input means and controlling the operation of said image read-out section according to the determined read-out condition.
Since the above-described radiation image recording and read-out apparatus is generally used for diagnosing human bodies, it is desired that the image obtained by this apparatus have high diagnostic efficiency and accuracy to perform effective diagnosis. The desirable output density or the range of the density level or contrast of the finally obtained visible image resulting from this apparatus depends upon the purpose of the diagnosis or the sort of the recorded portion of the body or the kind of disease.
Therefore, in order to obtain desirable images, it is desired to read out the image information stored in the stimulable phosphor sheet in the read-out step under the appropriate conditions particularly suitable for the purpose of diagnosis.
Further, since the above-described radiation image recording and read-out apparatus has a function to improve the diagnostic efficiency and accuracy by conducting a proper image processing on the image signal obtained by the read-out step, it is desired to make the image processing suitable for the diagnostic purpose. The image processing includes various kinds of contrast control, density level control, image gradation control, frequency processing, unsharp masking process and any other image processing steps having an effect of enhancing the diagnostic efficiency and accuracy, and also includes special technique of image processing such as image subtraction.
The above-mentioned read-out conditions and the image processing conditions suitable or desirable for the particular diagnostic purposes are determined according to the recorded portion of the object (head, chest etc.), the method of recording (simple recording or angiography or tomography etc.), purpose of diagnosis (inspection of bone shapes or condition of blood vessels etc.), which are generally called "kind of recording", and also according to "exposure conditions" of the radiation such as the tube voltage, tube current, exposure time, size of focus of the radiation source which are determined or controlled in view of the kind of recording.
As a method of determining the desirable read-out condition and the image processing condition, it is known to make a preliminary read-out of the image information stored in the stimulable phosphor sheet and determine the desirable conditions based on the radiation image information obtained thereby. The preliminary read-out is conducted by use of stimulating rays of lower energy level than that of the final read-out. This method, however, necessitates two read-out steps and takes a long time for read-out, which is not efficient in case of diagnosing a number of people where a high speed of image recording and read-out is demanded. Further, since the stimulating rays are used for the preliminary read-out, the life of the stimulating ray source is shortened. Furthermore, by the increase of the number of scanning, the life of the stimulable phosphor sheet is shortened, too.